The levels of noise and speech signals propagating through an in-service transmission path are used to determine the quality of the path. For example, the transmission quality of the path may be questionable if the level of the noise signals is high, or if the level of the speech signals is low (weak). It can be appreciated, therefore, that an accurate determination of the quality of a transmission path requires an accurate measurement of the levels of noise and speech signals as they propagate through the transmission path.
Prior measuring arrangements have been unable to achieve such accuracy, since they do not accurately measure the level of noise present in a transmission path. For example, one such prior measuring arrangement performs multiple measurements over a respective 30 millisecond window when noise signals are believed to be present on the transmission path. Noise signals are believed to be present when the level of the signal on the path falls below a predetermined threshold. At that point, the prior measuring arrangement measures the level of all signals that are present on the transmission path within the 30 millisecond window. The arrangement then determines an average noise value for the window. When the prior measuring arrangement has completed a number of such measurements, it then outputs, as the noise level of the transmission path, the average having the lowest value. This prior measuring arrangement does not provide an accurate noise level measurement because both speech signals and noise signals could be present during the 30 millisecond window, thereby biasing the resulting average.
U.S. Pat. No. 5,216,702 to the inventor hereof, the specification of which is incorporated herein by reference, describes a method and apparatus for obtaining a nonintrusive measurement of noise and speech signals present on in-service connection by first determining whether samples of signals received from the connection correspond to noise or speech. The noise and speech levels of the connection are then determined as a function of such samples. Specifically, individual average power levels are determined for succeeding groups of noise signals that are present on the connection within a predetermined period of time. An average power level is then determined for the succeeding groups as a whole. When a predetermined number of such average power levels have been determined, the median of such average power levels is output as the noise measurement of the connection. A speech level measurement is obtained in a similar manner.
The median of the average power levels was chosen as the output in the method and apparatus of U.S. Pat. No. 5,216,702 because it provided an accurate measurement of noise level during testing of the system under laboratory conditions. Under laboratory conditions, the testing environment is generally well controlled because noise and speech are injected into the system from recorded sources. However, when the system was tested in the field subsequent to the issuance of U.S. Pat. No. 5,216,702, it was discovered that a median of the average power levels does not provide an accurate measurement of noise. Specifically, a significant amount of ambient background noise, such as a television or a baby, which is absent under laboratory conditions, exists in the field. This background noise adds significant variability to the signals present on the transmission path. This variability due to background noise cannot be discriminated from the circuit noise present on the channel, and thus a median of the average power levels tends to overstate the actual circuit noise level of the channel.